measurement of solid liquid interface temperature during pulsed excimer laser melting of polycrystalline silicon films

The interface velocity has been studied by the transient conductance measurements,2 while various transient temperature mea-surement techniques have been developed.3–5 Measurement of int

Measurement of solid–liquid interface temperature during pulsed excimer

laser melting of polycrystalline silicon films

Xianfan Xu and Costas P Grigoropoulos

Department of Mechanical Engineering, University of California, Berkeley, California 94720

Richard E Russo

Energy and Environmental Division, Lawrence Berkeley Laboratory, Berkeley, California 94720

~Received 13 May 1994; accepted for publication 27 July 1994!

A nanosecond time resolution pyrometer has been developed for measuring the transient

temperature of thin polycrystalline silicon ~p-Si! films irradiated by a pulsed excimer laser The

sample design structure and material optical properties allow direct measurement of the temperature

at the solid–liquid phase change interface © 1994 American Institute of Physics.

Surface melting of semiconductor materials by pulsed

excimer lasers has been studied extensively in the literature

In most cases, rapid melting and resolidification induced by

the pulsed excimer laser irradiation is understood as a

one-dimensional process The excimer laser irradiation offers a

well controlled experimental technique for the study of the

interface response when the local equilibrium conditions are

disturbed The interface response function is often described

by the interface kinetic theory1 where it is shown that the

solid–liquid interface superheating is approximately

propor-tional to the velocity of the interface:DT5CVint, where C is

a material constant This interface kinetic relation is widely

adopted in the literature for numerical simulation of rapid

melt propagation To verify the kinetic relation and quantify

the material constant C, both the interface velocity and the

interface temperature need to be determined The interface

velocity has been studied by the transient conductance

measurements,2 while various transient temperature

mea-surement techniques have been developed.3–5 Measurement

of interface temperature has also been reported.6,7 However,

rather than being able to measure the temperature right at the

phase change interface, these methods measure the

tempera-ture at a certain distance away from the interface,6 or the

response due to the temperature-dependent material

proper-ties integrated over a certain depth.7 The accuracy of these

methods relies largely on accurate knowledge of material

properties Here we report the measurement of the interface

temperature based on the transient thermal emission

mea-surement at the solid–liquid interface with a nanosecond

time resolution ~notice that the fastest pyrometer reported is

of 2ms time resolution8!

The sample structure is a 0.5mm thick p-Si film

depos-ited on top of a 0.5 mm thick fused-quartz substrate, by

low-pressure chemical-vapor deposition The p-Si film is

heated by a pulsed KrF excimer laser with a pulse duration

of 26 ns and a wavelength of 248 nm ~Fig 1! A beam

homogenizer is used to ensure spatial uniformity in the laser

beam The laser intensity uniformity on the sample surface is

measured to be within 10% over the central 90% portion of

the laser beam spot The laser spot size on the sample surface

is about 6 mm2 A germanium diode is used to detect the

emission signals As shown in Fig 1, thermal emission from

the sample is measured from the back side of the sample, in

the wavelength range between 1.1 and 1.7mm In this wave-length range, both the solid silicon and the quartz substrate are transparent, so that the emissivity of these materials is zero according to Kirchhoff’s Law In contrast, the liquid silicon has an emissivity about 0.28 Thus the entire mea-sured thermal emission signal can be ascribed to liquid sili-con At near-IR wavelengths, the liquid silicon has a radia-tion absorpradia-tion depth less than 18 nm Therefore the measured thermal emission comes from the liquid in the im-mediate vicinity of the solid–liquid interface The effect of the movement of the interface on the energy collection

~depth of field effect! is negligible This is because the maxi-mum melting depth achieved in this experiment is less than 0.4 mm, which is five orders of magnitude smaller than the focal lengths of the lenses ~65 mm!

The germanium diode senses the thermal emission from

an area of 1 mm2 at the center of the heated spot It is re-versely biased to achieve a rise/fall time of 1 ns The electric signal from the germanium diode is recorded on a digitizing oscilloscope with a 1 GHz sampling rate Bandpass filters with center wavelengths at 1.2, 1.4, 1.5, and 1.6 mm and bandwidths of approximately 0.08 mm are used to acquire the spectral thermal emission signals The emissivity of the sample is obtained from a transient reflectivity measurement

FIG 1 Experimental setup for transient thermal emission and emissivity

measurements during pulsed excimer laser melting of p-Si films.

A quartz halogen~QH! lamp is used as the light source The

light of the QH lamp ~Fig 1! is focused onto the sample

surface with a beam spot size of about 1 mm2 The

reflectiv-ity of this beam is refocused by lenses onto the germanium

diode and recorded by the oscilloscope Bandpass filters~the

same as those used in emission measurement! are used to

measure the reflectivity at different wavelengths Two

mea-surements are taken at each wavelength and laser fluence,

one for the thermal emission measurement ~with the QH

lamp off! and one for the reflectivity measurement which

also includes the emission signal The reflectivity is obtained

by subtracting the thermal emission signal from the total

signal To eliminate the effect of the laser energy instability,

the fluence of each laser shot is measured The experimental

data are accepted when the measured laser fluence has the

desired value To eliminate the effect of the laser energy

instability The surface of the sample shows little change

from shot to shot for the range of laser fluences used in this

experiment

Planck’s distribution of blackbody radiation intensity

law is used to derive the temperature from the measured

thermal emissions:

e lb5 2pC1

where e lb is the blackbody emissive power, and C1 and C2

are blackbody radiation constants The detector collects

ther-mal emission within a solid angle ~u1tou2,f1tof2! and a

wavelength bandwidth ~l1 to l2! The voltage signal

re-corded on the oscilloscope, V, is expressed as

V 5W/pEl 1

l 2 Eu 1

u 2 Ef 1

f 2

e8~l,u,f,T!t~l!D~l!

In the above equation, W is the impedance of the

oscil-loscope ~50 V!,t~l! the spectral transmission of the lenses

and filters, D(l) the responsivity of the germanium diode ~in

units of A/W!,e8~l,u,f,T! the directional spectral emissivity,

and dA the area on the sample where the thermal emission is

sensed Invoking Kirchhoff’s law, the directional spectral

emissivity is equal to the directional spectral absorptivity:

For an opaque material~such as liquid silicon!, the

spec-tral directional absorptivity can be expressed as

a8~l,u,f,T !512R s~l,2u,f,T !2R d ~4!

R s~l,2u,f,T! is the specular reflectivity while R d is the

diffuse reflectivity It is assumed that the melt propagation

front is planar, and the diffuse reflectivity at the melt front

can be neglected Thus the emissivity can be obtained from

the specular reflectivity measurement:

e8~l,u,f,T !512R s~l,2u,f,T! ~5!

The temperature is obtained by solving Eq.~2! from the

thermal emissions measured at four different wavelengths

The temperature measurement is calibrated at steady state

with a quartz halogen lamp, whose temperature is measured

by a NIST calibrated pyrometer The confidence level in

de-termining the absolute temperature value is estimated to be

650 K The error in determining the relative temperature

~temperature difference at different time or at different flu-ence!, which is determined by the resolution of the digitizing oscilloscope, is615 K

Figure 2 shows thermal emission signals at the wave-length of 1.5 mm The thermal emission measurement also yields the melting duration ~indicated by arrows!, since only liquid silicon emits light in the wavelength range between 1.1 and 1.7 mm Comparing the thermal emission signal at

F50.55 and 0.65 J/cm2

, it can be seen that the maximum interface temperature increases with the laser fluence How-ever, when the laser fluence is higher than 0.65 J/cm2, the maximum interface temperature does not increase with the laser fluence The effect of a temperature gradient at the solid–liquid interface is considered in calculating the inter-face temperature From numerical simulation,9 it is found that the temperature gradient at the interface~about 3.3 K/nm

at a laser fluence of 0.95 J/cm2! causes the measured tem-perature to be approximately 40 K higher than the actual interface temperature The maximum solid–liquid interface temperatures at different laser fluences are shown in Fig

3~a!

In our previous work,9 the transient melting front posi-tion was measured using the transient conductance method The transient melting depth was calculated using a heat con-duction model which incorporates a melting front tracking algorithm and the interface response function The optical

refractive index of the p-Si film at the excimer laser

wave-length was measured using a variable angle spectral

ellip-someter, which is n˜exc51.212.8i Thermal and optical

prop-erties of bulk silicon10,11 and quartz10 were used in the calculation The measurement matched well with the numeri-cal simulation for quantities such as melt depth and melt

FIG 2 Transient thermal emission signal at l51.5 m m.~a! F50.55 J/cm2 ,

~b! F50.65 J/cm2

,~c! F50.75 J/cm2

, and~d! F50.95 J/cm2

.

duration Here we compare the melt duration obtained from

the thermal emission measurement with numerical

simula-tions @Fig 3~b!# Good agreement has been achieved

be-tween the measured melt duration and the calculations The

possibility that the high-temperature solid silicon can

con-tribute to the thermal emission measured from the back side

of the sample is examined by measuring the surface

reflec-tance The surface reflectivity measurement also yields the

melting duration due to a large reflectance increase of silicon

upon melting The melting durations measured by surface

reflectance are in close agreement with those obtained from

thermal emission measurement Considering that any

pos-sible thermal emission from the high-temperature solid

sili-con can last much longer than the melting duration, we can

dismiss the possibility that thermal radiation is emitted from

high-temperature solid silicon

The calculated maximum melt front velocities at

differ-ent laser fluences are shown in Fig 3~c! A comparison

be-tween the interface velocity and the interface temperature

@Figs 3~a! and 3~c!# allows us to determine the coefficient C

in the interface response function Assuming that there is a

linear relation between the interface superheating tempera-ture and the interface velocity at fluences lower than 0.65 J/cm2, the response function coefficient C is determined to

be around 6 K/~m/s! When the laser fluence is higher than 0.65 J/cm2, the interface superheating temperature is ‘‘satu-rated’’ at about 110 K In some cases, the maximum melt depth is used instead of incident energy as an indication of the actual energy coupled to the material Calculation of maximum melt depth versus laser fluence is presented in Fig

3~d!

Thermal emission from the top of the sample is also measured to verify that the surface temperature does not reach the boiling temperature ~2628 K! In the case of sur-face evaporation, the solid–liquid intersur-face velocity in-creases only slightly with fluence; the excess laser energy is consumed by the latent heat of vaporization The experimen-tal results show that the maximum surface temperature at the laser fluence of 0.95 J/cm2 is about 2100 K, well below the boiling temperature of liquid silicon

Support of this work by the National Science Founda-tion, under Grant No CTS-9210333, is gratefully acknowl-edged R Russo acknowledges the support by the U.S De-partment of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract No DE-AC03-76SF00098 The authors also want to acknowledge the help of Andrew C Tam of IBM Almaden Research Cen-ter and Hee K Park during the experimental work, and Xiang Zhang for sample preparation

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FIG 3 ~a! measured maximum interface temperature ~b! Comparison

be-tween the measured and calculated melting duration ~c! Calculated

maxi-mum melting velocity and ~d! calculated maximum melting depth at

differ-ent laser fluences.